English

Noun

triple point ‎(plural triple points)

1. (physics) The unique temperature and pressure at which the solid, liquid and gas phases of a substance are all in equilibrium.
   • 2006, David B. Troy & ‎Paul Beringer, Remington: The Science and Practice of Pharmacy, ISBN 0781746736, page 179:

     A solid sublimes only when the pressure of its vapor is below that of the triple point for that substance.

A pressure vs. temperature diagram showing phases of matter and the triple point

1. (physics) A point where the incident shock wave, the reflected shock wave, the Mach stem, and one slipstream meet.
   • 2007, Gabi Ben-Dor, Shock Wave Reflection Phenomena, ISBN 3540713824, page 198:

     When the incident shock wave is at position “a” the second triple point, T, is still inside the zone where the vibrational degree of freedom has not yet reached its equilibrium and the dissociational degree of freedom is still frozen at that position, as its excitation can be assumed to start only after vibrational equilibrium is reached.

   • 2012, George Emanuel, Shock Wave Dynamics: Derivatives and Related Topics, ISBN 1466564202, page 81:

     A triple point may have a convex Mach stem, in which case the SS points upward.

   • 2015, George Emanuel, Analytical Fluid Dynamics, Third Edition, ISBN 1498715702, page 130:

     One constant, however, is that the flow, just downstream of the incident shock, at a triple point, must be supersonic.

2. (mathematics) A point where a curve or surface intersects itself along three arcs.
   • 1998, J. Scott Carter & ‎Masahico Saito, Knotted Surfaces and Their Diagrams, ISBN 0821805932, page 162:

     The number of the distinguished edges whose orientations point towards the triple point is exactly one, or exactly two.

   • 2007, Rida T Farouki, Pythagorean-Hodograph Curves: Algebra and Geometry Inseparable, ISBN 3540733973:

     A cubic surface with a triple point (a cubic cone) may or may not be rational.

Boy's surface, showing the triple point where it intersects itself

1. (meteorology) The intersection of an occluded front, cold front, and warm front.
   • 1993, Meteorological and Geoastrophysical Abstracts:

     A further investigation is made of the reasonably-close relationship between some multiple tornado outbreaks in the British Isles and the passage of frontal-system triple points (TPs) using, principally, TORRO records and the hourly British Isles charts archived by the Meterorological Office at Bracknell.

   • 2002, Joseph M. Moran, Online weather studies, ISBN 1878220500, page 232:

     ...together at the point of occlusion, or triple point, where conditions are favorable for formation of a new cyclone, sometimes called a secondary cyclone.

   • 2013, Phil Croucher, Private Helicopter Pilot Studies JAA BW, ISBN 0978026942, page 8-34:

     The point where occluding starts is the triple point, because there are three air masses involved: that ahead of the occlusion, the warm air aloft, and the air behind the occlusion, or where the cold, warm and occluded fronts meet.

2. (meteorology) The point where two boundaries, such as a dry line, outflow boundary, or cold front, intersect.
   • 1947, Maurice Adolph Garbell, Tropical and Equatorial Meteorology, page 205:

     Few, however, reach the African continent; some dissipate in the intertropical-front bulge east of southern Africa; others form triple points with the temperate fronts available for recurving anywhere between Australia and the Seychelles-Madagascar-Mascarenes (Reunion-Mauritius-Rodriguez) sector.

   • 2006, The International Journal of Meteorology:

     It is concluded that the severe storms formed in close proximity to a "triple point" (TP), at which a synoptic cold front and "insolation-enhanced" warm front intersected.

3. (plate tectonics) A point where three plates intersect.
   • 2003, Neville Price, Major Impacts and Plate Tectonics, ISBN 0203165454, page 131:

     Such a 'piecrust model' has been used to explain the generation of triple points which can give rise to splitting of a continental body.

   • 2012, O. Oncken & ‎C. Janssen, Basement Tectonics 11 Europe and Other Regions, ISBN 9400915985, page 43:

     Microstructures like static mineral growth, equilibrated grain boundaries with triple points especially in quartz and feldspar fabric and the lack of plastic crystal lattice deformations even in quartz indicate that metamorphism outlasted deformation.

   • 2014, Duncan Keppie, The Analysis of Diffuse Triple Junction Zones in Plate Tectonics and the Pirate Model of Western Caribbean Tectonics, ISBN 1461496160, page 24:

     End-member stable triple point configurations are indicated in which the triple point is fixed to the North or South American plates (dashed grey lines) or fixed to the Caribbean Plate (dotted grey lines).

triple point in East Africa, where the Arabian Plate, and two parts of the African Plate (the Nubian and the Somalian) are splitting along the East African Rift Zone.

Related terms

• melting point
• boiling point

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